1. Field of the Invention
The present invention relates to a drive shaft bearing structure, and more particularly to a drive shaft bearing structure for watercraft.
2. Description of the Background Art
As seen in FIG. 7 of the present application, a drive shaft bearing structure for watercraft of the background art has been known. In addition, Japanese Patent Laid-open No. Hei 7-112697, the entirety of which is hereby incorporated by reference, describes a drive shaft bearing structure of the background art. As seen in FIG. 7, a driven-side coupler 2 connected to a driving-side coupler of an engine (not shown) is fixed to the front end of a drive shaft 1. An impeller (not shown) is fixed to the rear end of the drive shaft 1. Power from the engine is transmitted to the impeller, whereby the impeller is rotationally driven.
The bearing structure includes a bearing body 4 for rotatably supporting the drive shaft 1 relative to a watercraft body 3. The bearing body 4 includes a bearing member 5 (in the structure shown, a ball bearing) rotatably supporting the drive shaft 1, a bearing chamber 6 for containing the bearing member 5, and seal members 7, 7 disposed on both sides (front and rear sides in the axial direction) of the bearing member 5. The seal member 7, 7 partition the bearing chamber 6 by making contact with the outside circumferential surface 1a (in the structure shown, the outside circumferential surface of a sleeve 1′ rotated together with the shaft 1) of the drive shaft 1.
The present inventors have determined that the background art suffers from the following problems. In such a bearing structure, the bearing body 4 is mounted on the watercraft body 3 in the condition where the bearing chamber 6 is filled with grease. This is for imparting a smooth rotation of the bearing member 5. Where the bearing body 4 and the drive shaft 1 are mounted on the watercraft body 3, the seal members 7, 7 make contact with the outside circumferential surface 1a of the drive shaft 1 to partition the bearing chamber 6 as described above, and the bearing chamber 6 is maintained basically airtight. Therefore, unless the drive shaft 1 is rotated (hence, the watercraft runs), the grease charged in the bearing chamber 6 will not easily leak out of the bearing chamber 6.
However, the drive shaft 1 is actually rotated by the driving of the engine, and the impeller is rotated to move the watercraft, so that the temperature inside the bearing chamber 6 is raised due to the rotation of the drive shaft 1 and the bearing member 5. When the temperature in the bearing chamber 6 is raised, the grease and/or air in the bearing chamber 6 expands. According to the bearing structure as described above, an end portion 7a of the seal member 7 is disposed toward the outside of the bearing chamber 6.
Therefore, when the grease or air in the bearing chamber 6 expands over a predetermined level, the grease or air in the bearing chamber 6 leaks out to the exterior through a gap formed between the contact portion 7b for the outside circumferential surface 1a of the shaft 1 of the seal member 7 and the outside circumferential surface 1a of the shaft 1. The leaking out of the grease or air is due to deformation of the seal member 7 arising from the rise of the pressure inside the bearing chamber 6.
Accordingly, even when the grease or air in the bearing chamber 6 expands, excessive deformation of the seal member 7 due to this expansion is obviated. However, the leaking out of the grease in the bearing chamber 6 from the bearing chamber 6 due to the temperature rise in the bearing chamber 6 and the attendant expansion of the grease and air in the bearing chamber 6 can be problematic. A watercraft generally cycles or repeats through periods of running and stopping, whereby the temperature in the bearing chamber 6 (and hence expansion and contraction of the grease or air in the bearing chamber 6) rises and falls repeatedly. When the temperature is lowered, air penetrates into the bearing chamber 6 through minute gaps and clearances. When the temperature is raised, the air expands to push out the grease to the exterior of the bearing chamber 6. Therefore, the grease in the bearing chamber 6 is reduced in amount in a comparatively short time as the watercraft repeatedly stops, starts and operates.
Accordingly, and depending on the running patterns of the watercraf, it is necessary to charge the grease into the bearing chamber 6 at comparatively short intervals in the bearing structures of the background art. Accordingly, maintenance intervals and effort is increased with the systems of the background art.